The present invention relates to an oil cooler, for example, which cools engine oil or working oil of automatic transmission (ATF) for a vehicle.
Conventionally, the oil cooler arranged inside of a tank of a radiator has plural tubes stacked with each other, and oil flows through the tubes. Heat exchange is performed between oil passing through the tube and cooling water passing outside of the tube, so that oil is cooled. Moreover, an inner fin is arranged inside the tube, and promotes the heat exchange between oil and cooling water.
Such oil cooler secures suitable radiation performance by increasing the number of the tubes, as a heat emitting amount is required to be increased. Here, a size of the radiator tank is inevitably decided by a size of the oil cooler. In order to make the radiator tank to be thin and light, it is necessary to reduce the stacking number of the tubes by improving radiation performance per tube in the oil cooler.
By the way, as a kind of the inner fin, an offset fin is known (for example, refer to JP-B2-4240136) other than straight fin and wavy fin. The offset fin is usually used for a different use from the oil cooler, for example, for a heat exchanger which cools exhaust gas in an exhaust gas recirculation device (hereinafter referred as EGR cooler), or an intercooler.
However, physical properties of fluid to be cooled by the oil cooler are different from those to be cooled by the EGR cooler or the intercooler, as to be mentioned below. Therefore, dimensions of fin pitch fp, fin height fh, and segment length L of the conventional offset fin for the EGR cooler or intercooler cannot be directly applied to the oil cooler.
That is, in the oil cooler, a flow velocity of oil in the tube is as low as about 0.2-0.4 m/s. It is used in a range where the Reynolds number is as small as about 20-40 (laminar-flow region) when a diameter of a corresponding circle is defined as a representative length. Dynamic coefficient of viscosity has high temperature dependency. Further, since the Prandtl number of oil is as large as 100 or more, heat transfer phenomenon is different from the EGR cooler or intercooler which cools air.
Moreover, due to the above-mentioned physical properties of oil, a thickness of thermal boundary layer which influences heat transfer performance becomes very thin in the oil cooler. By reducing (decreasing) a fin pitch fp up to reaction limit of effect that tears the thermal boundary layer, heat transfer area is increased so as to promote the heat transfer.
If the conventional offset fin for the EGR cooler or intercooler is simply applied to the offset fin for the oil cooler, heat emitting properties of the oil cooler may be lowered.